Microsoft is making some tweaks to its browser strategy for Windows 10 that center on the rendering engines for its two browsers: Internet Explorer 11 and the one codenamed Project Spartan, which it sees as "the future."

The rendering engine is at the heart of a browser. It processes the under-the-hood programming-level instructions in order to create a Web page or run a Web app.

Back in January, Microsoft's plan of record was to include both its existing "Trident" (MSHTML) and its new "Edge" rendering engines in both IE 11 and Spartan. That way, whether customers were using IE 11 or Spartan, they'd be able to switch back to the legacy Trident engine if and when they encountered legacy or enterprise sites that needed Trident to render correctly.

But based on feedback from Windows Insiders and customers -- who've been able to test the new Edge engine even though Spartan itself isn't yet available in publicly available test builds of Windows 10 or Windows 10 Mobile -- Microsoft is shifting gears.

Microsoft has seen developers opt into the new Edge engine more rapidly than expected. Relatively few sites seem to require the fallback to the legacy engine.

Company officials have said that Spartan will be the default browser in all versions of Windows 10. They have not stated publicly which versions of Windows 10 will ship with IE 11 preinstalled, but I hear any device running the Windows 10 "desktop" (as opposed to Windows 10 Mobile) version will have IE 11 shipped as part of the operating system.

Both browsers are important components of Windows 10. But Spartan is the future, as Microsoft execs stated in today's blog post:

"Project Spartan is our future: it is the default browser for all Windows 10 customers and will provide unique user experiences including the ability to annotate on web pages, a distraction-free reading experience, and integration of Cortana for finding and doing things online faster. Web developers can expect Project Spartan's new engine to be interoperable with the modern Web and remain 'evergreen'with no document modes or compatibility views introduced going forward."